The objective of this study is to identify how the inflammatory cytokine, interferon gamma (IFN-?, which is up regulated in response to muscle injury resulting from trauma or disease, modulates muscle function and repair. The repair of skeletal muscle is essential to human health and inefficient repair leads to a loss of locomotion and eventually, to death. The inflammatory response plays an important role in responding to injury and initiating skeletal muscle repair. IFN-? plays both positive and negative roles in myogenesis and is required for efficient muscle regeneration in vivo. We have discovered that IFN-? acts through the class II transactivator, CIITA, by repressing the expression or activity o myogenin, the Myogenic Regulatory Factor (MRF) required for myofiber formation. In our studies to understand the mechanism of CIITA repression, we have discovered that IFN-?, through CIITA, acts to maintain the expression of the polycomb complex, PRC2, which is normally silenced during differentiation. CIITA recruits the PRC2 complex to repressed gene promoters and PRC2 represses gene expression by catalyzing the methylation of histone H3, lysine 27 (H3K27me3). We propose that transient expression of IFN-?, through CIITA, modulates myogenesis and promotes muscle repair, but suggest that deregulation of this signaling contributes to muscle disease by altering the gene expression profile in myofibers subject to IFN-? stimulation. In this proposal, we will determine how IFN-? and CIITA repress myogenin by determining if recruitment of activating factors such as MyoD are blocked in the presence of IFN-?, defining the epigenetic modifications used in the repression and determining whether the PRC2 complex is required for repression. Next, we will determine if genes known to be altered in H3K27me3 patterns upon differentiation are methylated in response to IFN-? and dependent on myogenin. These data will be correlated with the recruitment of EZH2 and the repression of gene expression. Lastly, we will confirm our findings in vivo and determine if chronic IFN-? can inhibit muscle repair. This work will utilize a new mouse model engineered to express low chronic levels of IFN-?. Using this mouse model, we will characterize the expression of CIITA and the PRC2 complex, determine if IFN-? target genes are deregulated, and characterize repair in the presence of chronic IFN-?. To understand the impact of IFN-? as part of the inflammatory infiltrate, the methylation profile and gene expression of IFN-? regulated gene targets will be assayed in mdx mice, which are known to have enhanced IFN-? expression as part of a chronic inflammation response. The proposed work will strengthen our understanding of the molecular events that occur during inflammation in skeletal muscle. This information will not only help elucidate the normal process of repair and regeneration, but also extend our knowledge about the potential deleterious effects of chronic inflammation, enhancing the understanding of diseases such as muscular dystrophy, cachexia and inflammatory myopathies and allowing the development of innovative therapies for these diseases.